Rewrite SnippetsFinder to improve performance

What does this MR do?

Summary: this MR rewrites SnippetsFinder from the ground up, improving its
performance by a factor of about 1500 (in the best case scenario).

This completely rewrites the SnippetsFinder class from the ground up in order to
improve its performance. The old code was beyond salvaging. It was complex,
included various Rails 5 workarounds, comments that shouldn't be necessary, and
most important of all: it produced a really poorly performing database query.

As a result, I opted for rewriting the finder from scratch, instead of trying to
patch the existing code. Instead of trying to reuse as many existing methods as
possible, I opted for defining new methods specifically meant for the
SnippetsFinder. This requires some extra code here and there, but allows us to
have much more control over the resulting SQL queries. It is these changes that
then allow us to produce a much more efficient query.

To illustrate how bad the old query was, we will use my own snippets as an
example. Currently I have 52 snippets, most of which are global ones. To
retrieve these, you would run the following Ruby code:

On GitLab.com the resulting query will take between 10 and 15 seconds to run,
producing the query plan found at https://explain.depesz.com/s/Y5IX. Apart from
the long execution time, the total number of buffers (the sum of all shared
hits) is around 185 GB, though the real number is probably (hopefully) much
lower as I doubt simply summing these numbers produces the true total number of
buffers used.

The new query's plan can be found at https://explain.depesz.com/s/wHdN, and this
query takes between 10 and 100-ish milliseconds to run. The total number of
buffers used is only about 30 MB.